In order to reset admin's password on FortiGate OS you have to access the FortiGate unit using console cable.
1. Connect your PC to the FortiGate unit console port.
2. Reboot the FortiGate unit.
3. Wait for system start-up and press any key when prompt to enter recovery menu.
4. Login with username: maintainer password: bcpb+serial number
The serial number can be found on the bottom of the unit (FGTxxxxxxxxxxxx).
5. After successfuly login enter the command to change the admin's password:
config system admin